When signals, made up of multiple symbols arranged in blocks, are transmitted over a wireless communications channel, there is a high probability that the signal will be received with some form of distortion. For example, the signal may arrive at the receiver along multiple paths, such as a direct line-of-sight path and one or more reflected path. This distortion has the possibility to cause inter-symbol interference, or inter-block interference.
The Nyquist criterion sets the conditions that must be met by the channel, and by the transmitted symbols, in order to avoid inter-symbol interference. One aspect of the Nyquist criterion is that, for a given channel, it sets a lower limit on the time separation of the data-bearing pulses.
It is also known that it is possible to transmit signals in the form of Nyquist pulses, but with a time separation between pulses that is shorter than that specified by the Nyquist criterion. This is referred to as Faster-than-Nyquist signalling. This has the advantage that it can increase the data throughput. However, it has the disadvantageous effect that there will be inter-symbol interference, that is, that samples obtained at the receiver will be dependent on more than one of the transmitted symbols. There is therefore an increased probability of errors in the receiver.